Many techniques exist for the production of high quality silica sand and beneficiation methods are normally related to the level of contaminant in the raw material. Generally, the industrial processing routes follow a basic pattern and include the washing of sand by a variety of techniques. Essentially, the washing of sand is undertaken using simple settling cones, upward current washing or screw classifiers. Each of these techniques is normally employed to remove the finer fraction material and relies largely on differential settling based mainly on size distribution of mono-mineralic material. Further, as the product will be granular in a size range exceeding 75 .mu.m, settling will also remove the materials of low specific gravity such as clays and organic material.
Numerous methods for the further treatment of sand have also been utilized using chemical and mechanical means.
Chemical treatment methods generally comprise subjecting the sand to an acid treatment stage in which iron impurities are reacted with the acid and taken into solution.
British Patent Specification No. 449,184 discloses a method of purifying sand having free iron present as an impurity by dissolving the iron impurities in a sulphuric acid solution, (without the presence of hydrochloric acid) so that they may be removed by washing, in which the sand is heated in the acid solution within a closed vessel in which no free oxygen is present in the atmosphere or dissolved in the solution, the sand being wholly immersed in the acid solution which is heated to its boiling point.
British Patent Specification No. 651,702 discloses the treatment of sand having a ferruginous coating, comprising agitating the sand with a dilute aqueous solution of hydrofluoric acid containing hydrosulphurous acid or an at least sparingly soluble hydrosulphite and, in addition, pre- or post-treatment froth flotation, to remove heavy mineral contaminants.
Whilst the use of hydrofluoric acid containing a sparingly soluble hydrosulphite is an effective process for the removal of ferruginous coatings, in recent years hydrofluoric acid has become a most expensive reagent and thus is not cost effective when large quantities of sand require to be processed.
The mechanical means of purifying sand largely depends on attrition scrubbing whereby grain to grain abrasion takes place, causing the sand grain surfaces to be cleaned by reducing clay and iron minerals adhering there.
Virtually all sand deposits require some form of beneficiation to reduce the Fe.sub.2 O.sub.3 content to a level suitable for use in the preparation of colourless glass. Generally this level is about 0.03% Fe.sub.2 O.sub.3. As the deposits of high quality sand become depleted it is necessary to develop processes which are capable of treating raw sand with a higher level of impurities than that which has previously been processed to achieve glass-making sand specification.
Our copending UK Patent Application GB 2111035A discloses a process for treating sand which includes one or more stages in which the sand is subjected to mechanical attrition in the presence of zinc or sodium hydrosulphite and an acid selected from sulphuric acid and hydrofluoric acid at a solids content of at least 70% by weight and preferably by 80% by weight.
It has been found that by attrition scrubbing of the sand grains in the presence of zinc or sodium hydrosulphite and hydrofluoric acid or sulphuric acid, a quick and effective method of removing iron impurities from the sand is provided. The treatment stage may vary in time generally from 1 to 20 minutes and it has been found that the effect generally takes place during the first five minutes of the treatment. The treatment stage is referred to hereinafter as an "attreatment" stage. On completion of the attreatment stage, the sand is washed, eliminating iron in solution, and the attreatment stage may be repeated several times to increase the purity of the product.
The present invention provides a new and improved process which may be used for purifying raw sand to glass-making specification.